Fm-discriminator



June 14, 1960 E. CHRISTIAN 2,941,075

FM-DISCRIMINATOR Filed Feb. 24, 1959 uIsc. WITH HIGH FIG SENSITIVITY VOISCJ REAGTANCE CONTROLLED TUBE OSCILLATOR fif III vDIsc2 SERVO LG) oIsc. WITH LOW SYSTEM SENSITIVITY FIGS v oIsc.I v 0|$c.I

voIsc.

2 v.oIsc.2

5 voIsc.2

FIG.6

v DISC. I+ VDISC. 2

x I FIG 3 U fif 2 f f INVENTOR.

ERICH CHRISTIAN 5*w m QM ATTORNEY United States Patent 2,941,075 FM-DISCRIMINATOR Erich Christian, San Carlos, Califi, assignor to the United States of America as represented by the Secretary of the Army The invention described herein may be manufactured and used by and for the United States Government for governmental purposes, without the payment to me of any royalty thereon.

The invention relates in general to an FM-discriminator and more particularly to an FM-discriminator having a high sensitivity and a wideband response.

Discriminators in automatic frequency control (AFC) systems generate a D.C. voltage, the polarity of which depends on the frequency deviation of the input signal from the center frequency of the discriminator. The sensitivity and band of response of a discriminator are each dependent on the other. If it is desired to increase the sensitivity of a discriminator it is necessary to narrow the band of response of the discriminator, and vice versa, if it is desired to make the band of response of a discriminator wider it is necessary to decrease the sensitivity of the discriminator. This feature has disadvantages in AFC systems as it is desirable to have a discriminator having both high sensitivity and a wide-band response. I have discovered a discriminator circuit arrangement which has theadvantages of both high sensitivity and a wideband response.

The novelty of my circuit arrangement resides in the fact that a narrowband and a wideband FM-discr-iminator are designed around one vacuum tube. A radio frequency input signal is fed to the control grid of a plate circuit of which contains a narrowband discrimi-' nator. In series with the screen, grid are two tuned circuits, each having a different resonant frequency. A broadband discriminator is in coupling relationship to the two tuned circuits. There will be a frequency between the resonant frequencies of the two tuned circuits at which the series circuitwill resonate. This resonant frequency is'made equal to the center frequency of the narrowband discriminator which causes the screen grid to be substantially grounded when the frequency of theinput signal is in the neighborhood of the center frequency and the narrowband discriminator has maximum sensitivity. When the frequency of the input signal approaches the resonant frequency of one of the two tuned circuits a DC voltage is obtained from the broadband discriminator, the polarity of which depends on the sign of the frequency deviation from the resonant frequency of the series circuit. The narrowband and wideband discriminator voltages can be used separately to act on different AFC devices or combined to act on a single device.

It is therefore an object of my invention to provide a circuit arrangement having a narrowband and a wideband FM-discriminator designed around one vacuum tube.

Another object of my invention is to provide an FM- discriminating circuit having broad and sharp discriminating characteristics.

Another object of my invention is to provide a relatively simple discriminator circuit means which has broad and sharp discriminating characteristics and can be utilized in a frequency cout1' y It can also be noted from pentode, the

,Another object of my invention is to provide a circuit arrangement using two FM-discriminators in which near the nominal frequency one of the discriminators is operative and the other is substantially inoperative and when the deviation from the nominal frequency is large the said other discriminator is operative and the said one discriminator is substantially inoperative.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings in which:

Figure 1 shows typical discriminator curves;

Figure 2 is a schematic illustration of the circuit em bodying the invention;

Figure 3 shows the reactance curves of the tuned circuits in the screen grid circuit of the pentode in Figure 2;

Figure 4 illustrates how the two discriminator voltages of Figure 2 can beused to act on different AFC devices;

Figure 5 shows how the two discriminator voltages of Figure 2 can be combined to act on a single device;

Figure 6 shows a curve of the output voltage of Figure 5.

With reference to the drawings, Figure 1 shows two typical discriminator frequency voltage curves, one with high sensitivity and the other with low sensitivity. It can be noted from the curves, that in order to make a dis criminator more sensitive it is necessary to bring the two peaks of the discriminator curve closer together. This has the disadvantage that the DC. voltage output of the discriminator decreases rapidly with increasing frequency deviation of the input signal applied to the discriminator. the curves that if the two peaks of a curve are made to be further apart the sensitivity of the discriminator is decreased.

Figure 2 discloses a circuit arrangement in which the advantages of both high sensitivity and wideband response are attained. The pentode 1 has an input terminal 2 connected to its control grid, to which a radio frequency input signal is to be applied. The resistor and capacitor unit 3 connects the cathode and suppressor grid to ground. The plate of the pentode is connected through an LC circuit 4 to a power supply, and the LC circuit 4 is tuned to a center frequency f The LC circuits-5 and 6 are induc tively coupled to LC circuit 4 and are tuned to predetermined frequencies slightly above and slightly below the center frequency f The diodes 7 and 8 develop across their load resistors 9 and 10, voltages that vary with the frequency of the voltage applied to the input terminal 2. The voltages 'across resistors 9 and 10 are of opposite polarity; therefore, the voltage at terminal 11 is the difference between the voltages across resistors 9 and 10. The voltage at terminal 11 is applied to a low pass filter, consisting of resistor 12 and capacitor 13, for the purpose of decreasing any ripple in the voltage at terminal 11. The frequency input-voltage output curve of the discriminator in the plate circuit will take the form of the high sensitivity curve of Figure 1. I do not wish to limit my disclosure to the particular discriminator shown and described in the plate circuit as any discriminator with a high sensitivity characteristic could be used. The signal in the plate circuit can also be used, without applying it to a discriminator, by applying it to a low pass filter.

In series with the screen grid of pentode 1 are two LC circuits 14 and 15 which are tuned to resonant frequencies f, and f The frequency f is less than the resonant frequency of the LC circuit 5 and 6 which is tuned to a frequency slightly below the center frequency f and the frequency f is greater than the resonant frequency of the LC circuit 5 or 6 which is tuned to a frequency slightly above the center frequencyf 'Ihe inductances of the two L r i s 4 nd 15 a e h P m e of two r a come 'less effective.

formers; is shown in Figure 3. When the frequency of the input vvoltage is below f and increasing, the reactanceof the tWofLG circuits inr's eries isiinductiveahd increasing. Whenthe frequencyis-f oneiof the twoZLC circuits will 7 have a very high resistivegimpedance; therefore, the impedancegfatheseries circuit will be very-high. As the frequency;increases-beyond f the reactance of the'LC- The reactance curv s of the screen grid circuit I .7

- to withoutideparting from the spirit andfthe scope of the that numerous changes in the details ofconstruction and the combination and arrangement of parts may be resorted invention as hereinafter claimed.

circuit; tuned to '13 will be capacitive'and the reactance of .7

the LC circuit tuned to f will be inductive. The reactance of the series circuit will be capacitive until: the freqencyQreaches ,a frequency 7 at which frequency the tive reactance of the'LC circuit tuned to will-"be equal to the capacitive "rea'cta'nce: of: the LC circuit tuned to h. As 'Illbgfl'SQllliCY, increases beyond f the reactance of the series circuit: will be inductive of frequencies for'producin'g a'voltag'e which varies about:

lllltlll the frequency reaches -f at which 'ifrequency I the LC circuit tuned to f willhave a very highresistive impedance; therefore, the series circuit wilhhave a very high impedance. As the frequency increases beyond f the reactance of the' series circuit-becomes capacitivefand decreasing. '7 Y V i =Thevoltages induced in transformer secondaries '16 and 17 by the transformer primaries of the LCcircuits 14 and 15; will be combinedthrough diodes 18 and 19 v and-will appear across resistors 26 andll. Potentiometer 22 has a slider 23-which is adjusted to azero potential which the input signal applied-to terminal 2 is at frequency i When the trequency of the inputsignal deviates from What is claimed is:

1. A frequency discriminatorihaving both sensitivityand a wideband frequency' response comprising a vacuum tube having a plate, a cathode, a control grid and v lscreen" grid; means connected ito 'said control grid for feeding" a radio frequency input signal to saidfvacunm tube, means connected to the plate of said tube and tuned to a predeterminedfrequency, meanscoupl ed with said last named means and responsive, over a narrow'band a predeterrr'iined value as "the; frequencyiof; the voltage in the plate circuit varies about'thepredetenriined frequency, means connected to the screen grid of said tube and tuned to the said predetermined frequencyandlres ponsive over a wide band of' frequencies, means coupled with said last namedmeans for producing avoltage which varies about the said predetermined value as the the frequency a voltage-will appear at slider'23 which will be representativefdf the frequency deviation from f The frequency input-voltage output curve, of the discriminator in the screen grid circuit-will take the form ofthelow sensitivityfcurve ofFigure l. I do not wish to. limit my invention to the particular discriminator circuit' shown and described in the screen grid circuit as others could be-uscd." Forinstance, the-LC circuits '14 and 15 could betwo inductances connected in series and a discriminator such asshown in the plate circuitcould be used by tuning LC; circuits -5 {and 6 to resonant frequencies f {and f and inductively coupling the two LC 'circuitstto;the.-tvvo .inductances 7 Frequen y f is made'equal to frequency i a-nd;the op-V erationtof the circuit'of Figure 2 is; as followsz ;.'A n input signal is applied to the grid of pentode l throughterminal V lf thejfrcquency ot the input signalisclose :tolth'e center frequency f -of the discriminatorin the pla'te circuitof thepentode the reactance of theseries circuit in the screen v grid.qvill -be;very low, theji-tube 1 acts as a pentodeiand the'nari ow band discriminator in; the plate circuitihas mairimum sensit ivity. If the frequency vdc"- viation from f becomes larger either the;.tun ed circuit frequency of the voltage intthe grid circuit varies about -the predetermined frequency and means'for combining over a narrow band offrequencies for producingafvolb age which variesabout a predcte'rmined value as the frequency. o f'the voltage inthe plate circuit varies about apredeterminedfrequency means'connected'to the screen gridlo'f saidtube and responsive over a widebandoffre- 'quencies for producing a voltage which varies about the" said predetermined value as the frequencyof the voltage -t e r n a d. c cuit r cs b u t e ai tivity and atwideb'and frequenc response'asclaimed in r a H claim V ZLinwh'ich the said'mea'ns connected to: the screen predetermined frequency and means for combining the voltages, produced by the two s aid means j for ;producing a'voltage; to provide a voltage which varies overia wide band of I iqi encies and sharply inzthe -region of said preaaermineat equene {i o; :1" s:- A vfrequencyjdiscrinhnatortfilim .beth high sensi- ZcOnapri Ses two'lparallel jiugl lctiyc-c g p d v? i u t-S, connectediin series a dtuned to frequencieswhose difier- 'ence'is-greater than said narrow bandandsuch that the impedanceof the two parallel circuits connected in series Morithetuned circuit wil'il reasonate causing the, dis'e a ciimiilator in the screen gridcircuitjto. become :mor'e efiective and. the discriminatorin the plate circuitlto bee Figure 4s ows how the voltages, 'V g 1 "and-V5 i the joutput'voltages of the discriminators in Figurei2, can

be jutilizedseparat'ely. to control an oscillator. One of the voltages controls'the oscillator through a reactance through a servoisystem.

tube and the other voltage is used to control the oscillator Figura -5 shows ho- {t e Y ag nvincxrif: lld:vane; can be combined to acton a single device i 'I'he 'two voltages are added through resistors R and R such that,

The graph ofgthe'voltage at the outputoflthe circuit in Figure 5 is ,shownby Figurer6." Theoutput of thercircuit of Figurefi be usedito controlQan oscillator. a Although-Ihavefiescribedi my" invention with aicertain f degree od particularitmfit understood that the present V a disclosure has been made only by way of example and grid :Qiig a aric about fire ua x-st 1;: so: 1 1 7 t I 5. 1m a frequency control system as claimedr in, claim- 4 inwhich the said means connected to the screen grid is 'fzero latfthelsaid predetermined frequency and means coupled to said twoinductive-capacitive circuits for'producingfa voltagewhich varies about the said predeterm ned a a h eque cy o the v a ei n th c ee 1 gridncircuit varies aboutthe' said predetermined frequency. 4.Q:I1 r a frequencycontrol-system, a vacuum 'tubehavring 'a-plate' a control grid and a screen grid, ,meanscon nected .to said control grid for feeding a radio frequency puL signal to said vacuum tube,zmeans connected to plate. of said tube and responsive overa predetermined band of frequencies for producing a voltage which varies about a predetern'iinedlvalue 'a spthe frrcquency of the voltage in the plate circuit varies about a predetermined frequency, means connected to, the screen grid of said tube and responsive over a band of frequencies greater t an ai 'p e e m a d band at t eaue a s mducin g a voltage which varies about the s'aid predeterminedyalueas thefrequencyofthe-voltage the screen 7 the said, predetermined comprises two parallel inductive-capacitive circuits connected in series and tuned to frequencies whose difierence is greater than said predetermined band and such that the impedance of the two parallel circuits connected in series is zero at the said predetermined frequency and means coupled to said two inductive-capacitive circuits for producing a voltage which varies about the said predetermined value as the frequency of the voltage in the screen grid circuit varies about the said predetermined frequency.

6. A frequency discriminator having both high sensitivity and a wideband frequency response comprising a vacuum tube having a plate, a control grid and a screen grid, means connected to said control grid for feeding a radio frequency input signal to said vacuum tube, means connected to the plate of said tube and responsive over a predetermined range of frequencies for producing a voltage which varies about a predetermined value as the frequency of the voltage in the plate circuit varies about a predetermined frequency, a first parallel inductivecapacitive circuit connected to the said screen grid and tuned to a frequency which is less than the lower limit of said predetermined range, a second parallel inductivecapacitive circuit connected in series with said first inductive-capacitive circuit and tuned to a frequency which is greater than the upper limit of said predetermined range and such that the total inductance of the two inductivecapacitive parallel circuits is zero at said predetermined frequency, a first diode and resistor connected in series and inductively coupled across said first parallel inductivecapacitive ci cuit, a second diode and resistor connected in series and inductively coupled across said second parallel inductive-capacitive circuit, and means for combining the voltages across the two resistors and the voltage produced by the second mentioned means.

Crosby July 1, 1947 Robinson Sept. 18, 1951 

